Fluids at cryogenic temperatures, also referred to herein as “cryogenic fluids”, include liquefied gases that can have boiling points below −100° C. (about −150° F.) at atmospheric pressure. Examples of such fluids include liquefied natural gas (LNG) and other gases, such as nitrogen, oxygen, carbon dioxide, methane and hydrogen that are storable in liquefied form at cryogenic temperatures.
A problem with known storage tanks that store cryogenic fluids is that heat leak into the storage space can cause vaporization of some of the stored liquefied gas causing the pressure within the tank to rise above the relief pressure set point, reducing the time that liquefied gases can be held within the tank without venting to relieve the pressure. It is generally preferable to avoid venting any fluid since, among other reasons. This results in a loss of the stored fluid, instead of the lost fluid being delivered to the use device. In this disclosure, vapor is defined as a substance in the gas phase at a temperature lower than its critical temperature, which means that a vapor can be condensed to a liquid or converted to a solid by increasing the pressure, without reducing the temperature.
As an alternative to venting into the atmosphere, by way of example, the cryogenic delivery and storage system illustrated in U.S. Pat. No. 5,421,161 shows an economizer circuit that includes a line connecting the vapor space in the tank to a delivery line that supplies fuel to the use device. When the pressure in the tank rises above a predetermined level a regulator included in the economizer circuit opens the flow of vapor from the vapor space inside the tank to the use device. By taking vapor from the vapor space, the pressure inside the tank falls. The system further comprises a line for delivering liquid fuel from the liquid space of the tank to the use device through a relief valve and a vaporizer. The vaporizer converts the cryogenic fluid stored in the tank into the gas phase so that gas can be delivered to the use device and the relief valve provides a fixed back pressure in the liquid fuel delivery line. When the regulator in the economizer circuit is open, more vapor flows from the vapor space inside the tank because the back pressure created in the liquid delivery line creates a path of least resistance through the economizer circuit. This system allows a relatively fast drop in pressure in the tank when needed, although it does present the disadvantage that the predetermined value of the tank pressure which triggers the opening of the regulator in the economizer circuit can not be adjusted. Another disadvantage is that the relief valve on the liquid delivery line is susceptible to freezing because the liquefied gas, which is at cryogenic temperatures, circulates therethrough.
Another example of an economizer circuit for reducing the pressure in a cryogenic tank without venting vapor into the atmosphere is described in U.S. Pat. Nos. 6,125,637, 6,494,191, 6,619,273, 6,953,028 and 7,044,113. These patents describe an economizer circuit that comprises a so-called economizer valve that is connected to a conduit extending from the vapor space of the tank and, also to a conduit extending from the liquid space of the tank. The economizer valve selectively withdraws either liquefied gas or vapor from the tank depending on the pressure within the vapor space inside the tank. The selected fluid, either liquefied gas or vapor, passes through a vaporizer disposed downstream of the economizer valve. The economizer valve described in U.S. Pat. No. 6,125,637 is configured to automatically operate in one of two positions for either withdrawing vapor or liquefied gas from the storage tank. In this respect this economizer circuit is similar to the one described before because it does not allow a gradual pressure decrease in the tank. The system is designed to automatically and passively maintain a predetermined pressure within the vapor space of the fuel tank without using any electrically actuated flow controlling devices. No adjustment of the predetermined range of the optimal pressure within the tank can be performed because the economizer valve is set to switch from one position to the other at a predetermined pressure.
The economizer valve described in U.S. Pat. Nos. 6,494,191, 6,953,028 and 7,044,113, although similar to the one described in U.S. Pat. No. 6,125,637, is different in that it allows a gradual drop in pressure inside the storage tank because it enables an intermediate position of the valve member such that both liquefied gas and vapor can be delivered from the storage tank at the same time. Also, the valve member can be actuated by a solenoid and therefore the liquefied gas and vapor delivery times can be adjusted.
Another example of adjusting the pressure inside a cryogenic storage tank is described in U.S. Pat. No. 6,334,312. Gaseous medium is supplied from the cryogenic tank to the consumer until the pressure inside the tank drops, at which time the supply is switched to liquid medium. The liquid medium is vaporized in a heat exchanger placed outside of the storage tank before it is delivered to the consumer. The gaseous and liquid medium withdrawal lines are joined into a common line. The switching of fuel supply between the gaseous and liquid mediums is done through a valve that is disposed on the common line inside a vacuum insulated area of the storage tank or inside the storage tank. The valve is actuated by an electrochemical actuator. The heat exchanger for vaporizing the liquid medium is disposed downstream of the valve. While the risk of freezing the valve is recognized as a potential problem, the proposed solution is to place the valve and its electrochemical actuator inside the storage tank, which is very expensive and does not allow an easy access to the valve for replacement or maintenance purposes.
In all of the embodiments illustrated in the previously mentioned patents the economizer valve is placed upstream of the vaporizer in the line that delivers the gas to a use device. This creates challenges in operating an economizer valve under optimum conditions because of the potential for freezing. Accordingly, there is a need for a system and a method of maintaining the pressure inside a storage tank for holding fluids at cryogenic temperatures within the predetermined values while preventing the freezing of the system components.